The present disclosure relates generally to downhole tools for neutron well logging and, more particularly, to neutron detector configurations for such downhole tools.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Downhole tools for neutron well logging have been used in oilfield settings for many years to measure formation porosity and as gas and lithology indicators. These downhole tools have historically included a radioisotopic neutron source, such as AmBe, which emits neutrons into the surrounding formation. The neutrons may interact with the formation before being subsequently detected in neutron count rates by one or more neutron detectors. Among other things, the neutron count rates may be sensitive to hydrogen in formation pore spaces. As such, the neutron count rates may be employed to determine a porosity of the formation.
Unfortunately, besides hydrogen in the formation pore spaces, detector count rates are also sensitive to other borehole and formation properties, collectively referred to as environmental effects, such as borehole size and fluid salinity. The magnitude of these effects depends on detector spacing as does the porosity sensitivity. Relatively speaking, the former are more significant compared to the latter at shorter spacings. Traditional designs employ this fact by determining porosity from the ratio of a near and far detector count rates. By deriving porosity from such a ratio, a number of undesirable effects such as the former are substantially reduced, albeit at the loss of some porosity sensitivity.
Moreover, in some instances, a radioisotopic neutron source may be undesirable for a variety of reasons. For example, the use of a radioisotopic source may involve negotiating burdensome regulations, the sources may have limited useful lives (e.g., 1 to 15 years), and the strength of the sources may need monitoring. Moreover, radioisotopic sources are becoming more expensive and more difficult to obtain. When alternative neutron sources, such as electronic neutron generators, are used in place of a radioisotopic neutron source, the response of the neutron detectors may not enable traditional neutron porosity determination. This may occur because the higher neutron energy of an electronic neutron source may produce a dramatic loss in porosity sensitivity, and hence measurement quality, at high porosities.